Foamed product and manufacturing method of the same

ABSTRACT

A foamed product that generates no smoke when incinerated, has a low calorific value, and achieves a desired dimension and thermal insulation performance is provided. A foamed product  1  is formed by heat-mixing, in an extruder  11 , a paper pellet  7  that is a mixture of a fine paper powder having a particle diameter of 30 to 200 μm and starch, a container recycle pellet  8  that is a mixture of polypropylene, polyethylene, and polystyrene, foamable polypropylene  9 , and water  10 . A weight distribution is set so that the paper pellet  7  is 50 to 65% by weight, the container recycle pellet  8  is 13 to 25% by weight, the foamable polypropylene  9  is 10 to 30% by weight, and the water  10  is 10 to 20% by weight to these resins and other materials.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a foamed product suitable for a heatinsulator, a shock absorber, and the like, and to a manufacturing methodof the foamed product.

2. Description of the Related Art

Currently, various types of foams and the like are used as heatinsulators or cushioning media. For example, as a heat insulator forresidential thermal insulation, a mineral-based heat insulator such asglass wool or rock wool, a plastic-based heat insulator such as rigidurethane foam or extruded polystyrene foam, or a heat insulator using anatural ingredient such as cellulose fibers or carbonized foam cork isput to use.

Patent Document 1 listed below discloses, as a residential heatinsulator, a plate-like heat insulator that is fit between supportmembers such as floor joists, pillars, and ceiling joists and composedof a foamed styrene resin molding or a rigid polyurethane foam.

-   [Patent Document 1] Japanese Patent Application Laid-Open No.    H08-74345-   [Patent Document 2] Japanese Patent Application Laid-Open No.    2003-41041

SUMMARY OF THE INVENTION

The heat insulator disclosed in above-mentioned Patent Document 1 is aplastic-based heat insulator, which has a significant environmentalburden upon manufacture or disposal. For instance, a foamed styreneresin molding has a large carbon number due to its molecular structure.Accordingly, remaining carbon that cannot react with oxygen burnsincompletely and becomes soot, causing generation of smoke duringincineration. Besides, foamed polystyrene is about 9000 to 10000 cal/gin calorific value, that is, has a high calorific value per unit weight.

Moreover, these heat insulators are made fire-retardant by mixing afire-retardant material into raw materials, which causes emission oftoxic gas when burned. This raises a possibility of damage by toxic gasin the case of fire. Furthermore, since these heat insulators are highin calorific value, there is a possibility that an incinerator isdamaged upon disposal.

On the other hand, in Patent Document 2, the present inventors provide afoam member that generates no smoke when incinerated and has a lowcalorific value, and a manufacturing method of the foam member. If sucha foam member can be employed as a residential heat insulator, a heatinsulator without a significant environmental burden upon manufacture ordisposal can be obtained.

In view of this, the present inventors attempted to extrude the foammember disclosed in Patent Document 2, by an extruder having a widthdimension for a residential heat insulator. In detail, polypropylene, afine paper powder of 30 to 100 μm in average particle diameter, andstarch of 5 to 150 μm in average particle diameter were supplied to theextruder and mixed together under heat, and further water was mixed intothe high-temperature molten mixture to manufacture the foam member. Aweight ratio of polypropylene, the fine paper powder, and starch is suchthat polypropylene is 25% by weight, the fine paper powder is 55% byweight, and starch is 25% by weight. A weight ratio of water to themolten mixture is 25% by weight.

However, molding the foam member disclosed in Patent Document 2 by theextruder resulted in a failure to attain a desired width dimension. As aresult of investigating the cause of the failure, the present inventorsfound the cause to be an insufficient foam volume as a foamed product.

The present inventors then used foamable polypropylene suitable for afoamed product, as polypropylene which is a resin raw material of afoamed product. As a result, a sufficient foam volume was attained,enabling a foamed product of a desired dimension to be obtained.However, the obtained foamed product lacked sufficient thermalinsulation performance, and was unable to be used as a residential heatinsulator.

The present invention has an object of providing a foamed product thatgenerates no smoke when incinerated, has a low calorific value, andachieves a desired dimension and thermal insulation performance.

As a result of further conducting intense study, the present inventorsfound that desired foaming and them al insulation performance can beachieved by mixing, at a predetermined ratio, a recycled resin which isa mixture of a polyethylene resin, a polypropylene resin, and apolystyrene resin, into foamable polypropylene which is a resin rawmaterial of a foamed product.

A foamed product according to the present invention is a foamed productformed by tightly connecting surfaces of a plurality of rod-likeelements, each of the rod-like elements having, on a surface thereof, askin layer including no bubbles, the skin layer covering an inside foamlayer, the skin layer being 15 to 25 μm in average thickness, therod-like elements being formed from a heated and foamed mixture of apaper pellet of 50 to 65% by weight, a recycled resin of 13 to 25% byweight, and a foamable polypropylene resin of 10 to 30% by weight, atotal sum thereof being 100% by weight, the paper pellet being a mixtureof 35 to 50% by weight of a fine paper powder having a particle diameterof 30 to 200 μm and 50 to 65% by weight of a hydrophilic macromolecule,a total sum thereof being 100% by weight; the recycled resin being amixture of 55 to 65% by weight of a recycled polypropylene resin, 30 to40% by weight of a recycled polyethylene resin, and 1 to 5% by weight ofa recycled polystyrene resin, a total sum thereof being 100% by weight.In the present specification, the term “the total sum thereof being 100%by weight” allows inclusion of minute unavoidable impurities, inaddition to the listed materials.

With the above-mentioned range of weight distribution of each rawmaterial, the foamed product according to the present invention canachieve a desired dimension and thermal insulation performance as aresidential heat insulator. Moreover, the foamed product can bemanufactured using, as the recycled resin, a container recycle pellet(hereafter abbreviated as “container re-pellet”).

The container re-pellet is a pellet formed in order to reuse foodcontainers or daily commodity containers (e.g., shampoo containers,detergent containers, and so on) after cleaning and the like, andinevitably includes polypropylene, polyethylene, and polystyrene.Polypropylene and polyethylene are both a polyolefin-based resin, whichis considered to emit no toxic gas when burned. Hence, polypropylene andpolyethylene are extensively used for food containers and dailycommodity containers. However, despite an effort to mix the two resins,they do not mix with each other, so that the mixture cannot be molded.

Therefore, though the container re-pellet that inevitably includes thetwo resins is produced in large quantity due to increased recyclingactivities in recent years, demand for the container re-pellet is lowbecause it cannot be used as a raw material in ordinary plastic moldingfactories. Besides, various additives are added to each resin accordingto container applications. This being so, when the container re-pelletis used as a molding raw material, a molded object will end up beingunstable in quality. For these reasons, the container re-pellet has notbeen used as a molding raw material.

The foamed product according to the present invention uses such acontainer re-pellet as a raw material, where polypropylene andpolyethylene are uniformly mixed, with there being no unevenness inquality. Moreover, though desired performance as a heat insulator cannotbe attained when simply using foamable polypropylene as a raw material,the use of the container re-pellet enables a foamed product of highthermal insulation performance to be obtained.

Thus, the foamed product according to the present invention includes, asa raw material, at least 50% by weight of the paper pellet having a lowcalorific value, and so is low in calorific value. In addition, thepresent inventors conducted a burning test on the foamed productaccording to the present invention, and found no toxic smoke. Moreover,the foamed product according to the present invention can achieve adesired dimension and thermal insulation performance. Furthermore, sincethe container re-pellet which has conventionally been low in demand canbe used as a raw material, container recycling can be promoted.

As a result of mixing each raw material at the above-mentioned ratio andfoaming the mixture, the skin layer thickness was 15 to 25 μm on averagein the foamed product according to the present invention. In the case ofusing only foamable polypropylene mentioned above, the skin layerthickness was about 10 μm on average, and also the appearance wasclearly different. Hence, the foamed product according to the presentinvention exhibits high mechanical strength and high thermal insulationperformance.

Further, in the foamed product according to the present invention, therod-like elements are formed by heat mixing and foaming the paperpellet, the recycled resin, the foamable polypropylene resin and water,the water being in a range of 10 to 20% by weight to the paper pellet,the recycled resin, and the foamable polypropylene resin.

Moreover, in the foamed product according to the present invention, theskin layer and the foam layer are made by uniformly mixing the finepaper powder, the hydrophilic macromolecule, the recycled polypropyleneresin, the recycled polyethylene resin, the recycled polystyrene resinand the foamable polypropylene resin. For example, whether or not eachresin is uniformly mixed can be checked through analysis using aninfrared absorption spectrum.

Moreover, a manufacturing method of a foamed product according to thepresent invention is a method of manufacturing a foamed product bycharging a paper pellet, a recycled resin, a foamable polypropyleneresin, and water into an extruder and extruding charged materials from adie provided on an extrusion side of the extruder, the paper pelletbeing a mixture of a fine paper powder having a particle diameter of 30to 200 μm, a hydrophilic macromolecule, and water, and the recycledresin being a mixture of a polyethylene resin, a polypropylene resin,and a polystyrene resin, wherein the paper pellet includes 35 to 50% byweight of the fine paper powder and 50 to 65% by weight of thehydrophilic macromolecule, a total sum thereof being 100% by weight, therecycled resin includes 55 to 65% by weight of a recycled polypropyleneresin, 30 to 40% by weight of a recycled polyethylene resin, and 1 to 5%by weight of a recycled polystyrene resin, a total sum thereof being100% by weight, 50 to 65% by weight of the paper pellet, 13 to 25% byweight of the recycled resin, and 10 to 30% by weight of the foamablepolypropylene resin are charged into the extruder, the total sum thereofbeing 100% by weight, the paper pellet, the recycled resin, and thefoamable polypropylene resin are heat-kneaded in the extruder to form afluid, 10 to 20% by weight of the water is added to the fluid to causefoaming, and a result of the foaming is extruded from the die to moldthe foamed product, and upon extruding the result of the foaming fromthe die, rod-like elements are each formed to have a skin layer withoutbubbles and a foam layer covered with the skin layer, where adjacentrod-like elements are tightly connected to each other in a molten state.

By mixing the paper pellet, the recycled resin, and the foamablepolypropylene resin and extruding the mixture by the extruder in thisway, the foamed product in which each raw material is uniformly mixedcan be obtained even in the case where the recycled resin is used as araw material.

In addition, in the above-mentioned manufacturing method, upon extrudingthe result of the foaming from the die, rod-like elements are eachformed to have a skin layer without bubbles and a foam layer coveredwith the skin layer, where adjacent rod-like elements are tightlyconnected to each other in a molten state. This allows skin layerswithout bubbles to be tightly connected to each other in a molten state,so that the foamed product of high mechanical strength and high thermalinsulation performance can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view showing a state where a part of a foamedproduct according to the present invention is cut out.

FIG. 2 is a partially enlarged schematic view showing a state of a cutsurface in FIG. 1.

FIG. 3 is an explanatory view showing an extruder used in amanufacturing method of the foamed product according to the presentinvention.

FIG. 4 is a photograph showing a cross section and a surface of a foamedproduct which is an example of the present invention.

FIG. 5 is a photograph showing a cross section and a surface of a foamedproduct which is a comparative example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following describes a foamed product and a manufacturing method ofthe foamed product according to the present invention, with reference toFIGS. 1 to 4.

FIG. 1 is an explanatory view showing a foamed product 1 in anembodiment of the present invention. As shown in FIG. 1, the foamedproduct 1 in this embodiment is a plate-like foamed product formed byconnecting a plurality of rod-like elements 2 extending in an axialdirection. Though FIG. 1 shows a partially enlarged view of the foamedproduct 1 in this embodiment, the foamed product 1 is actually 1 m inwidth, 1 m in length, and 30 mm in thickness.

Each rod-like element 2 has a two-layer structure of a skin layer 3 anda foam layer 4, as shown in FIGS. 1 and 2. A large number of bubbles 5exist in the foam layer 4. The rod-like elements 2 are in a state whereskin layers 3 on their surfaces are tightly connected to each other.Moreover, there are voids 6 a between the rod-like elements 2, forminggrooves 6 b on the surface of the foamed product 1, as shown in FIG. 1.Main raw materials of the foamed product 1 in this embodiment are apaper pellet 7, a recycled resin pellet 8, and a foamable resin pellet9, with water 10 being used for foaming (see FIG. 3).

The paper pellet 7 is manufactured by mixing 35 to 50% by weight of afine paper powder obtained by grinding used paper into a fine powder,and 50 to 65% by weight of starch which is a hydrophilic macromolecule,a total sum thereof being 100% by weight. However, minute unavoidableimpurities are also included therein. When doing so, additives such asan antioxidant and a fungicide are added according to purposes andapplications. For example, as the used paper, roll paper which hasbecome industrial waste is coarsely grinded to about 2 mm square, andthen finely grinded to 30 to 200 μm in particle diameter using a solidroller mill not shown. As the starch, typically used industrial starchis employed.

The recycled resin pellet 8 is a container re-pellet obtained byrecycling containers made of synthetic resin. Though the recycled resinpellet 8 slightly differs in component ratio depending on the recycledcontainer type, in this embodiment the recycled resin pellet 8 is amixture of 55 to 65% by weight of polypropylene, 30 to 40% by weight ofpolyethylene, and 1 to 5% by weight of polystyrene, a total sum thereofbeing 100% by weight, and is molded in pellet form. However, minuteunavoidable impurities are also included therein.

The foamable resin pellet 9 is a foamable polypropylene in thisembodiment. Polypropylene typically has poor tensile strength in amolten state and so has low foamability. Accordingly, polypropylene isnot suitable for use in a foamed product. In recent years, however,foamable polypropylene with strengthened melt tension (so-called “highmelt strength polypropylene” or HMS-PP) has also been developed. As wellunderstood by persons skilled in this art, for example, as evidenced inU.S. Pat. No. 6,596,814 and U.S. Patent Application Publication No.2007-0004861 A1, such high melt strength polypropylenes are typicallydefined as having a melt tension of at least 5 centiNewtons (cN) at 230°C. In this embodiment, “NEWFOAMER” made by Japan PolypropyleneCorporation is used as one such foamable polypropylene.

In this embodiment, a ratio of these main raw materials is set in arange where the paper pellet 7 is 50 to 65% by weight, the recycledresin pellet 8 is 13 to 25% by weight, and the foamable resin pellet 9is 10 to 30% by weight (the total sum thereof being 100% by weight).Then 10 to 20% by weight of the water is added to these main rawmaterials. A foamed product having a foam expansion ratio of 30 to 50times is obtained by setting the ratio of the main raw materials in theabove-mentioned range in this embodiment. Since the foamed product 1 inthis embodiment has such a foam expansion ratio, a foamed product with awidth of 0.9 m to 1.0 m required as a residential heat insulator can bemolded.

The following describes a manufacturing method of the foamed product 1in this embodiment, with reference to FIG. 3. The foamed product 1 inthis embodiment is manufactured by charging the paper pellet 7, therecycled resin pellet 8, and the foamable resin pellet 9 into anextruder 11, adding water to these raw materials to cause foaming whilemelt-kneading them in the extruder 11, and adjusting the foamed productin thickness by a thickness adjustment roller 12.

The extruder 11 includes a hopper 13 into which the raw materials arecharged, a cylinder 15 having biaxial screws 14 inside, and a die 16provided at a downstream end of the cylinder 15. In addition, a watersupply line 18 for mixing the water 10 in a tank 17 with the mixtureinside the cylinder 15 is provided at an intermediate position of thecylinder 15. Moreover, a plurality of heaters 19 for heating the insideof the cylinder 15 are arranged in the cylinder 15 in an axial directionof the cylinder 15. Further, a motor 20 for rotating the screws 14 isprovided upstream of the cylinder 15.

The die 16 extrudes the molten raw material mixture, which is pushed outfrom an exit of the cylinder 15, to the outside from small openings notshown, thereby forming the foamed product 1. The thickness adjustmentroller 12 is provided downstream of a tip of the die 16. The thicknessadjustment roller 12 adjusts, by a distance between metal rollersdisposed on upper and lower sides and their pressing elastic force, thethickness of the object passing between the rollers.

The above-mentioned raw materials that are adjusted in supply amount arecharged into the hopper 13 of the extruder 11, from a raw materialsupply not shown. The raw materials charged into the hopper 13 aresupplied into the cylinder 15, and heated by the heaters 19 while beingstirred by the two screws 14 in the cylinder 15, thereby beingmelt-mixed.

In this state, the paper pellet 7, the recycled resin pellet 8, and thefoamable resin pellet 9 as the raw materials are kneaded with eachother, so that the fine paper powder and starch included in the paperpellet 7, recycled polypropylene, polyethylene, and polystyrene includedin the recycled resin pellet 8, and foamable polypropylene included inthe foamable resin pellet 9 are kneaded with each other. The rawmaterials in the cylinder 15 are melt-kneaded by the screws 14 whilebeing heated by the heaters 19, and conveyed downstream. During this,the molten mixture of the fine paper powder, starch, and the varioussynthetic resins is uniformly kneaded, as a result of which the finepaper powder is uniformly dispersed in the whole mixture.

Moreover, the heated water 10 is supplied from the water supply line 18,and mixed with the molten raw material mixture. The water 10 suppliedinto the raw material mixture is heated by the molten raw materials andthe heaters 19 and as a result vaporized, thereby forming a large numberof bubbles 5 in the raw material mixture.

The raw material mixture to which the water 10 is added in this way isthen extruded from the die 16. The raw material mixture extruded fromthe small openings of the die 16 not shown is released under atmosphericpressure. This causes the bubbles 5 in the raw material mixture toexpand, as a result of which the plurality of rod-like elements 2 areformed. The rod-like elements 2 are each formed to have the skin layer 3and the foam layer 4, when extruded from the small openings of the die16. Here, it is considered that the skin layers 3 of the adjacentrod-like elements 2 contact with each other, where the raw materialsforming the skin layers 3 are still in a molten state. Hence, theadjacent rod-like elements 2 are tightly connected to each other,thereby forming the foamed product 1.

After this, the foamed product 1 is cooled while being adjusted to aconstant thickness by the thickness adjustment roller 12, and conveyeddownstream. The foamed product 1 is then cut to a desired length.

Example

The following describes an example of the present invention. In thefoamed product 1 of Example 1, the fine paper powder in the paper pellet7 is 20% by weight and starch in the paper pellet 7 is 37% by weight, tothe whole foamed product 1. The recycled resin pellet 8 is 13% by weightto the whole foamed product 1. The foamable resin pellet 9 is 30% byweight to the whole foamed product 1. The paper pallet 7 also includesless than 1% by weight of “ADK STAB” made by ADEKA Corporation as anantioxidant, and less than 1% by weight of calcium from calcined scallopshells as a fungicide.

Roll paper treated as industrial waste is used as the raw material ofthe fine paper powder. Industrial starch in widespread use is used asthe starch.

The composition of the recycled resin pellet 8 itself is 60% by weightof polypropylene, 35% by weight of polyethylene, and 5% by weight ofpolystyrene. Further, 16% by weight of the water 10 is added to themixture of the fine paper powder, starch, and the various syntheticresins for foaming.

Further, in the foamed product 1 of Example 2, the fine paper powder inthe paper pellet 7 is 20% by weight and starch in the paper pellet 7 is35% by weight, to the whole foamed product 1. The recycled resin pellet8 is 15% by weight to the whole foamed product 1. The foamable resinpellet 9 is 30% by weight to the whole foamed product 1.

Still further, in the foamed product 1 of Example 3, the fine paperpowder in the paper pellet 7 is 20% by weight and starch in the paperpellet 7 is 35% by weight, to the whole foamed product 1. The recycledresin pellet 8 is 20% by weight to the whole foamed product 1. Thefoamable resin pellet 9 is 25% by weight to the whole foamed product 1.

Moreover, in the foamed product 1 of Example 4, the fine paper powder inthe paper pellet 7 is 20% by weight and starch in the paper pellet 7 is35% by weight, to the whole foamed product 1. The recycled resin pellet8 is 25% by weight to the whole foamed product 1. The foamable resinpellet 9 is 20% by weight to the whole foamed product 1.

In Examples 1 to 4, the total sum of the fine paper powder and starch inthe paper pellet 7, the recycled resin pellet 8, and the foamable resinpellet 9 is 100% by weight. However, minute unavoidable impurities arealso included therein.

FIG. 4( a) shows a state of a cross section of the foamed product 1 ofExample 1 and FIG. 4( b) shows a state of a surface of the foamedproduct 1 of Example 1. As shown in FIG. 4( a), the skin layer 3 has asmooth surface without bubbles. The thickness of the skin layer 3 isabout 20 μm on average, as a result of measurement based on FIG. 4( a).Meanwhile, the bubbles 5 in the foam layer 4 are 200 μm in averagediameter, as a result of measurement based on FIG. 4( a). As is clearfrom FIG. 4( a), most of the bubbles are closed cells.

In the foamed product 1 of Examples 2 to 4, the skin layer 3 also has asmooth surface without bubbles. The thickness of the skin layer 3 is 21μm in Example 2, 23 μm in Example 3, and 25 μm in Example 4. Further,the diameter of the bubbles 5 in the foam layer 4 of the foamed product1 of Examples 2 to 4 showed values equivalent to that in Example 1.Further, most of the bubbles in the foam layer 4 of the foamed product 1of Examples 2 to 4 are closed cells.

Moreover, components of the skin layer 3 and the foam layer 4 of thefoamed product 1 of Examples 1 to 4 were analyzed using an infraredabsorption spectrum at the Tokyo Metropolitan Industrial TechnologyResearch Institute which is a local independent administrative agency.As a result, it was found that the skin layer 3 and the foam layer 4both show the same spectral waveform, and have a uniform mixture ofpolypropylene, polyethylene, and polystyrene as the raw materials.

Thus, in the foamed product 1 of Examples 1 to 4, polypropylene andpolyethylene, which have conventionally been difficult to mix uniformly,are mixed uniformly. This demonstrates that, according to the presentinvention, a foamed product of stable quality can be obtained even whenusing, as a raw material, a container re-pellet which inevitablyincludes the two synthetic resins.

Furthermore, the foamed product 1 of Example 1 exhibits thermalconductivity of 0.035 W/mK on average when used as a heat insulator.Since the foamed product 1 has a thickness of 30 mm, i.e., 0.03 m,converting to thermal resistance yields 0.86. Thermal conductivitysimilar to Example 1 was obtained in each Examples 2 to 4.

Accordingly, the foamed product 1 of Examples 1 to 4 achieves thermalresistance of 2.29, when two foamed products of 30 mm in thickness andone foamed body of 20 mm in thickness are overlaid to have a thicknessof 80 mm. The foamed product 1 having such thermal resistance can meet arequirement (2.2) as a floor heat insulator for next-generationenergy-saving houses.

A comparative example in which only the foamable resin pellet 9 is usedas a raw material without using the recycled resin pellet 8 while theother raw materials and the manufacturing method are the same as Example1 is described below. As shown in FIG. 5( a), a foamed product 21 of thecomparative example has rod-like elements 22 that are each separatedinto a skin layer 23 and a foam layer 24 like the foamed product 1, butthe skin layer 23 is extremely thin with an average thickness of about10 μm.

Besides, as shown in FIG. 5( b), though there are no bubbles 25 on thesurface of the foamed product 21, the inside bubbles 25 are visible fromthe outside. An average diameter of the bubbles 25 in the foam layer 24as calculated based on FIG. 5( a) is about 100 μm.

Moreover, the rod-like elements 22 in the foamed product 21 are in closecontact with each other with almost no gaps, and there are almost novoids 26 a unlike the foamed product 1. Furthermore, grooves 26 b on thesurface are narrow and have a shallow depth, as compared with thegrooves 6 b of Example 1.

As a result of measuring the mechanical strength of the foamed product 1of Example 1, the foamed product 1 exhibits bending strength of 0.058MPa. Further, the foamed product 1 of Example 2 exhibits bendingstrength of 0.059 MPa, the foamed product 1 of Example 3 exhibitsbending strength of 0.062 MPa, and the foamed product 1 of Example 4exhibits bending strength of 0.064 MPa. On the other hand, the foamedproduct 21 of the comparative example has much lower bending strengththan in Examples 1 to 4. Thus, the foamed product 1 of Examples 1 to 4has high mechanical strength because the skin layers 3 of the pluralityof rod-like elements 2 are firmly and tightly connected to each other,as compared with the case of using only the foamable resin pellet 9.

This difference in mechanical strength is mainly attributable to thedifference in thickness of the skin layer 3 and the difference in degreeof tight connection between the rod-like elements 2. The average bubblediameter is about 200 μm in the foamed product 1 and about 100 μm in thefoamed product 21 of the comparative example. Superior thermalinsulation performance and mechanical strength of the foamed product 1of Examples 1 to 4 despite its larger average bubble diameter indicatesthat the thickness of the skin layer 3 plays a significant role.

In addition, even when a force is applied to separate each rod-likeelement 2 in the foamed product 1 of Examples 1 to 4, each rod-likeelement 2 is not easily separated. In the case of the foamed product 21of the comparative example, however, each rod-like element 22 can beseparated relatively easily.

Furthermore, in the foamed product 1 of Examples 1 to 4, the paperpellet 7 occupies at least 50% of the total weight of the foamed product1. Accordingly, the foamed product 1 itself is not subject to recyclingin the Law for Recycling of Containers and Packaging, and can bedisposed as general waste. Besides, calculating a calorific value of thefoamed product 1 of Example 1 yields about 5900 cal/g. The calorificvalue of the foamed product 1 of Examples 2 to 4 shows similar value.Since the foamed product 1 has such a low calorific value, there is nopossibility that an incinerator is damaged in the case of incinerationdisposal.

Though industrial waste roll paper is used as the fine paper powder inthe above embodiment, the raw material of the fine paper powder is notlimited to this, and may be any of various used paper such as oldnewspapers, old magazines, printed paper, used wrapping paper, usedcorrugated paper, used OA paper, broken paper or damaged paper duringmanufacture of virgin paper, and cutting wastage, abrasive powders,shredder scrap of magazines and the like. Waste paper is discharged inlarge quantity from offices, publishing companies, paper-manufacturingcompanies, and the like, and evaluated to have substantially noenvironmental burden. Note that the average particle diameter of thepaper powder is measured using a laser diffraction particle sizedistribution measurement device (Mastersizer S made by MalvernInstruments Ltd.).

Though starch is used as a hydrophilic macromolecule in the aboveembodiment, there is no particular limit on starch, as maize starch,sweet potato starch, potato starch, wheat starch, barley starch, ricestarch, and the like may be used. Moreover, the hydrophilicmacromolecule may be glue, natural rubber, or agar, instead of starch.Although not shown, it was confirmed that the similar result to those inthe above-mentioned examples is obtained, in the case where hydrophilicmacromolecules other than starch, such as a agar, is used.

Though the foamed product 1 has a dimension suitable as a residentialheat insulator in the above embodiment, the foamed product 1 is notlimited to use as a residential heat insulator, and may be used forvarious applications such as a shock absorber, a sound insulator, and acold insulator. In addition, the foamed product 1 may be molded invarious shapes according to applications.

Though “NEWFOAMER” made by Japan Polypropylene Corporation is used asfoamable polypropylene in the above embodiment, this is not a limit forthe present invention, and foamable polypropylene having similarproperties may be used.

What is claimed is:
 1. A foamed product formed by heat-mixing andfoaming: a paper pellet that is a mixture of 35 to 50% by weight of afine paper powder having a particle diameter of 30 to 200 μm and 50 to65% by weight of a hydrophilic macromolecule, a total sum thereof being100% by weight, wherein the hydrophilic macromolecule comprises one ofstarch, glue, natural rubber, and agar; a recycled resin that is amixture of 55 to 65% by weight of a recycled polypropylene resin, 30 to40% by weight of a recycled polyethylene resin, and 1 to 5% by weight ofa recycled polystyrene resin, a total sum thereof being 100% by weight;a high melt strength foamable polypropylene resin having a melt tensionof a least 5 cN at 230° C.; and water, wherein the paper pellet is 50 to65% by weight, the recycled resin is 13 to 25% by weight, and the highmelt strength foamable polypropylene resin is 10 to 30% by weight, atotal sum thereof being 100% by weight, the water is in a range of 10 to20% by weight to the paper pellet, the recycled resin, and the foamablehigh melt strength polypropylene resin, the foamed product, which has afoam expansion ratio of 30 to 50 times and a calorific value ofsubstantially 5900 cal/g, is formed by extruding charged materials froma die and passing the extruded materials between rollers to producetightly connected surfaces of a plurality of rod-like elements, each ofthe rod-like elements has, on a surface thereof, a skin layer having nobubbles, the skin layer covering an inside foam layer, the skin layer is15 to 25 μm in average thickness and exhibits an infrared absorptionspectral waveform equivalent to that of the foam layer, and voids areformed between the rod-like elements, the voids forming grooves onsubstantially planar exterior surfaces of the foamed product.
 2. Thefoamed product according to claim 1, wherein the skin layer and the foamlayer are made by uniformly mixing the recycled polypropylene resin andthe high melt strength foamable polypropylene resin with the recycledpolyethylene resin.
 3. The foamed product according to claim 1, whereinsaid foamed product exhibits a bending strength of at least 0.058 MPa.4. A method of manufacturing a foamed product, which has a foamexpansion ratio of 30 to 50 times and a calorific value of substantially5900 cal/g, by charging a paper pellet, a recycled resin, a high meltstrength foamable polypropylene resin having a melt tension of at least5 cN at 230° C., and water into an extruder and extruding chargedmaterials from a die provided on an extrusion side of the extruder, thepaper pellet being a mixture of a fine paper powder having a particlediameter of 20 to 200 μm, a hydrophilic macromolecule, and water, andthe recycled resin being a mixture of a polyethylene resin, apolypropylene resin, and a polystyrene resin, wherein the paper pelletincludes 35 to 50% by weight of the fine paper powder and 50 to 65% byweight of the hydrophilic macromolecule, a total sum thereof being 100%by weight, wherein the hydrophilic macromolecule comprises one ofstarch, glue, natural rubber, and agar, the recycled resin includes 55to 65% by weight of a recycled polypropylene resin, 30 to 40% by weightof a recycled polyethylene resin, and 1 to 5% by weight of a recycledpolystyrene resin, a total sum thereof being 100% by weight, 50 to 65%by weight of the paper pellet, 13 to 25% by weight of the recycledresin, and 10 to 30% by weight of the high melt strength foamablepolypropylene resin are charged into the extruder, the total sum thereofbeing 100% by weight, the paper pellet, the recycled resin, and the highmelt strength foamable polypropylene resin are heat-kneaded in theextruder to form a high-temperature fluid, 10 to 20% by weight of thewater is added to the fluid to cause foaming, and a result of thefoaming is extruded from the die to mold the foamed product, and uponextruding the result of the foaming from the die and passing theextruded materials between rollers, rod-like elements are each formed tohave a skin layer without bubbles and a foam layer covered with the skinlayer, where adjacent rod-like elements are tightly connected to eachother in a molten state, the skin layer is 15 to 25 μm in averagethickness and exhibits an infrared absorption spectral waveformequivalent to that of the foam layer, and voids are formed between therod-like elements, the voids forming grooves on substantially planarexterior surfaces of the foamed product.
 5. The method of manufacturinga foamed product according to claim 4, wherein said foamed productexhibits a bending strength of at least 0.058 MPa.